Be sure to check the hole for being off center.I bought 2 from tannery shop a few years ago that I dug out recently.The hole was drilled off center and I will have to use a small sanding drum to adjust the position before drilling.

This explains some of the problems between the cast and forged lowers that are out there (with pics no less), and his method will absolutely deliver a straight hole for the tap to cut the threads, and how to use a drill press to center and tap perfectly straight holes. The reaming method bases its centerline on the upper you'll be mating the lower to. Its about $50 worth of tooling (shell reamer, etc) if you shop around, but if you're building more than one its a worthwhile investment. Even so, you can always sell the tooling off after using it. I used this method on a National Ord cast receiver that had a buffer tower that was WAY off center. It came out perfect. Rifle runs like a champ.

"A large caliber is good to have, but its shot PLACEMENT that counts"
Synweap223

Use a fostner bit that is the same diameter as the hole that needs to be cut out I beleive 1 1/8th inch. It is as easy as pie with a jig. When you place the bit in the hole the bit will not spin unless it sits perfectley straight.